Stretchable electronics have the potential to enable a wide variety of emerging applications including sensors/electronic device integration into the textiles, conformable power, enhanced robotic mobility and manipulation, energy harvesting, fieldable biosensing, sensing technology, as well as exoskeletons and multifunctional conforming suits.
Over the past decade, significant effort has focused on the development of organic electronic materials for flexible display applications. However, these materials can only tolerate modest strains such as slight bending and will fail upon larger deformations.
Electrical conductivity in polymeric materials had typically been obtained by three methods:
1) Use inherently conductive polymers. These polymers typically consist of a conjugated backbone to provide electron transport. While the conjugated structure is beneficial for electrical conductivity, it creates an extremely brittle polymer that is prone to fracture at small strains. In addition, the conjugated structure dramatically increases the polymer glass transition temperature Tg making traditional melt processing impossible.
2) Incorporate conductive filler. A significant advantage of conductivity through the incorporation of conductive filler is that it can be readily implemented into a wide range of polymer matrices. The conductivity of the filled polymer composite will depend on the filler type, loading, dispersion, aspect ratio, and the interaction between the filler and the polymer. For example, conventional spherical filler materials can require loadings in excess of 30 vol % to obtain high conductivity.
FIG. 1 is a plot showing reduced viscosity as a function of loading for 200 nm spherical nickel particles in polystyrene (PS) demonstrating the exponential increase in viscosity with filler loading. Here, η* is the complex viscosity of the polymer-nickel composite melt and η*PS is the complex viscosity of the PS melt. At high loadings (e.g., greater than 0.3 vol. fract.), there is an inevitable impact on other properties of the composite including the uniformity of the electrical performance, elasticity, deformability, and processability.
High aspect ratios fillers can be used to produce percolating, conductive pathways at lower loadings than spherical fillers. Carbon nanotubes (CNTs) are the most prevalent large aspect ratio filler in the recent literature and have demonstrated electrical conductivity at low loadings in a variety of rigid, glassy polymer matrices and elastomeric materials.
Despite the favorable results, there are considerable disadvantages associated with practical implementation of CNTs including property uniformity (most promising single-walled CNTs are 66% undesirable semi-conducting CNTs), effective and scalable processing methods (most effective mixing through solution methods or even more complicated processes), and cost (speculated cost reduction of high purity CNTs has yet to be realized). In addition, long carbon nanotubes (typically utilized to produce electrical conductivity) are highly entangled which does not allow adjacent particles to move freely. As a result, conductive materials based on carbon nanotubes are flexible (tolerating modest bending and slight tension typically less than 5% strain) but are not stretchable (large deformations that can reach several hundred percent strain. Recent work has demonstrated that the entanglements between carbon nanotubes can be reduced using a secondary additive however; the composite exhibits increasing resistance with strain consistent with most conductive composites containing spherical particles. See, e.g., Lin et al., “Towards Tunable Sensitivity of Electrical Property to Strain for Conductive Polymer Composites Based on Thermoplastic Elastomer,” ACS Appl. Mater. Interfaces 2013, 5, 5815-5824. To make highly entangled carbon nanotube composites stretchable typically requires geometric patterning discussed in the section below.
3) Deposition of conductive materials on a flexible surface. Electronic devices that are formed from organic or inorganic conductive materials on thin plastic sheet or metal foils will be flexible but cannot typically undergo large deformations like stretching without damage.
FIG. 2 shows conventional examples of geometric patterning to obtain “stretchable” conductivity where FIG. 2(a) is a conductive carbon nanotubes mat that is perforated with a “diamond” pattern to enable deformation, FIG. 2(b) buckled ribbons, and FIG. 2(c) conductive “meanders” of metallic film. This past work incorporates geometric features that can tolerate slight stretching within a determined range. The production methods of these materials are rather intricate but could potentially be scaled up within the limits of current lithographic techniques. However, the incorporation of geometric features into circuits requires an additional finite element design step to avoid premature failure of the circuitry regardless of material.
Collectively, the existing conductive polymeric material, and recent advances in geometrically patterned devices will not meet future Army needs for stretchable electronics.